It’s all going to depend on what you’re trying to achieve. I mean, by the speed and position you probably could have gotten two or three more. I would definitely have stopped the set when you did, and quite possibly earlier because I generally call it when I start to slow down if it’s an AMRAP set.
Hell, some people get good results never really approaching failure. Some people get good results going as close to failure as they can. The common denominator shared by these results generating people is that they work hard all the time.
I think if you’re going to draw a line between when to push closer to failure and when to keep fairly distant from it, do it by whether you’re looking to primarily get stronger or primarily get bigger (of course they aren’t at all mutually exclusive). If you’re looking to mostly get stronger, you’ll probably be well served to steer relatively clear of failure while still making sure you do plenty of hard work. If you’re mostly looking to get bigger, you’ll probably do well to stay pretty close to failure although it isn’t essential provided you do enough hard work.
I tend to sit between the two. Because I compete in powerlifting, the last thing I want to do is get even vaguely accustomed to failing any main or supplemental lifts. Firstly, it’s a bad habit that can impact performance in meet and second, it places an unsustainable burden on recovery. On assistance work I’ll definitely approach failure quite regularly, because it doesn’t impact recovery as much and because those exercises don’t even remotely resemble my competition exercises. I don’t intend to approach failure, but if a set takes me there I go along with it.